But when we told them the test results revealed other implicit biases, such as one against career women, the elderly, blacks, Arabs and gays, some students got defensive.
Researchers say the test shows what's really in your subconscious, with sometimes surprising results. Not just young people but old people, too, showed an overwhelming bias against the elderly.
"You wouldn't expect that old people would think that old is bad. But the elderly are every bit as negative about the idea of old age as much younger people are," said Anthony Greenwald, a professor of psychology at the University of Washington.
The researchers also found about half of the black people who took this test showed bias against blacks.
Greenwald says many people discover they have biases that they wish they didn't.
"I certainly don't want to think of myself as a racist. But these things are in my head, they show up on the test," said Greenwald.
Of course, the biases in our head are only harmful if we act on them.
In 1999, four New York City police officers shot and killed Amadou Diallo, an immigrant from Guinea, while he reached into his pocket for what the officers feared was a gun but turned out to be his wallet.
Psychologists like Joshua Correll, a professor at the University of Chicago, study how race can affect such real-life decisions.
He has had thousands of people in America take a test in which different scenes flash across the screen. Then a person appears holding either something safe like a cell phone, or something lethal like a gun.
The results? "They shoot very quickly when an armed target is black. They take a little bit longer to shoot when the armed target is white," Correll said.
Even black people are quicker to shoot if the person is black.
"White, black, Latino, Asian, again and again and again, we see this same pattern of effects: bias in the reaction times, bias in the mistakes that they make," Correll said.
According to Correll, the more a test-taker tries to not appear biased the more bias shows up in the test results.
If we all have these subconscious biases. What can we do about them? Psychologists say we can control our conscious actions.
"If we're just aware that it exists, it gives us a chance to do something, to be vigilant to not let our unintended biases -- our implicit biases -- take over our behavior, which can happen unintentionally," Greenwald said.